You'll get the bonus against any opponent who wears an actual armour piece, or any who get similar stats because of their armour print. The only armour prints I could think of that are heavy enough are the KK I heavy plate print and the Shadowknight torso, though. There were a couple of soldiers with plated armour prints so far but they only had 4 armour so it didn't count against them, later though they might be.

Your character profiles have been updated, and Quantum of Surfer has joined the party as Alamar the Resilient. I have also decided to play as Barad the Ungiving, and used my first 10g towards light armour.